Agriculture Reference
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totally outweighted by the variation in climatic variables. To test these hypotheses, a long-
term experiment was established, where the parameters of microbial community were
examined in soil beneath an oak canopy and the nearest grassland, across seasons (Waldrop
and Firestone 2006). Oak canopies and grassland areas represented two different habitats for
soil microorganisms. The range of fluctuations of soil abiotic variables (temperature and
humidity) differs between these two vegetation types. Because of the bare soil surface they
are wider in grasslands and narrower in oaks. Due to the aforementioned differences,
microbial respiration and biomass were higher under oak canopies, resulting in similar
activity per unit of biomass in both vegetation types. The composition of microbial
communities was affected by the type of vegetation, the season and the year. Biomarkers for
Gram-positive, actinomycete and fungal groups characterized the oak samples, while
biomarkers for Gram-negative characterized the grassland samples. The magnitude of the
difference in microbial community composition attributed to vegetation type tended to be
similar or even less than the attributable to seasonal cycles. Moreover, an intra-annual
variability in microbial community composition was recorded in both plant communities. As
Waldrop and Firestone (2006) concluded, microbial biomass should be mainly controlled by
carbon inputs to soil while community composition by climatic variables.
And what about the relationship between composition and function? The functioning of
soil microbial community displayed seasonal fluctuations which related to soil microclimate,
but differences in function between oak and grassland soils were hardly detectable. Although
the function of the overall soil microbial community showed no differences among vegetation
types, functions mediated by specific groups of microbes respond differently (Waldrop and
Firestone 2004). Such functions could be the decomposition of simple and recalcitrant
compounds. Since the oak litter is a more complex C source in comparison with grassland
litter, it is likely that soil microbes beneath oak are able to degrade more recalcitrant C
compounds. Both in grassland and oak soils, the addition of substrates increased the response
of Gram negative
organisms. The incorporation of simple substrates (vanillin, starch and
xylose) enhanced the activities of the same groups of microbes in both soils. When pine litter
was added different microbial groups were activated showing that as the complexity of the
substrate increases, the groups responsible for its degradation were different in oak from those
in grassland soils. This supported the idea that the functional groups that degrade complex C
sources are not functionally redundant across ecosystems. So, any changes in relation to these
groups could have a great effect on ecosystem process. Similarly, the response of enzymatic
activities to the addition of simple compounds was identical between soils, despite differences
in the composition of microbial community. However, differences were recorded in
enzymatic activities regarding the degradation of the pine litter and that of the soil organic
matter.
The same line of experimentation followed the study of Fierer et al. (2003). They sought
for the response of the bacterial genetic diversity in oak and grassland soils to drying-
rewetting frequency. Soils of semiarid and arid Mediterranean-type ecosystems are
particularly susceptible to drying-rewetting stresses, due to the infrequency of rainfall events
and the warm-dry summer that enhance soil dryness. A rainfall event could induce lysis of
microbial cells releasing 30-60% of microbial C (Kieft et al. 1987). Most of the existing
information referred to enhanced rates of C and N mineralization as an immediate response to
rewetting of dry soils, while remained unexploited the question of how these repeated drying-
rewetting cycles affected the microbial community structure. Although a modification in
